Certain embodiments of the present invention relate to x-ray systems utilizing a solid state multiple element x-ray detector for producing an image, and in particular, to techniques and apparatus for identifying pixels susceptible to artifacts caused by excess pixel lag and for correcting the artifacts.
Solid state x-ray detectors are being developed that comprise a two dimensional array, typically of 1,000 to 4,000 detector elements in each dimension (x,y). Each detector element comprises a photo detector that detects and stores charge representative of an amount of radiation incident on the detector element. Each detector element further includes a thin film transistor (TFT) connected to the photo diode and operated as a switch to enable and disable read out of the charge stored on the photo diode. Each detector element ultimately produces an electrical signal which corresponds to the brightness of a picture element in the x-ray image projected onto the detector. The signal from each detector element is read out individually and digitized for further image processing, storage and display.
The solid state detector may be used in a variety of x-ray medical imaging applications. One such application is dual-energy imaging. In dual-energy imaging, two sequential x-ray images are acquired very close together in time. One acquisition is typically a low energy image (i.e. an image read out after 60-90 kVp exposure), while the other acquisition is typically a high energy image (i.e. an image read out after 110-140 kVp exposure). From the two raw input images, an algorithm is executed which creates a first xe2x80x9cbone-onlyxe2x80x9d image and a second xe2x80x9csoft tissue onlyxe2x80x9d image. This algorithm is known in the art and can take many forms, such as log subtraction. The combination of the two images enhances visualization of nodules and calcifications.
The solid state detector may also be used to acquire a greater number of sequential x-ray images. For example, a large number of images may be acquired to build a cine-loop of a heart. As with the dual-energy imaging described above, the acquisitions are acquired with a short time interval between them. The short time interval helps to reduce artifacts due to patient motion and/or capture the motion of the patient anatomy.
Solid state detectors unfortunately may experience image lag. Image lag is the retention in the detector of imaging information from prior images. For example, lag may be caused by residual charge in the photo diode or TFT of a detector element. The charge from a second or subsequent exposure is added to the residual charge, or lag, increasing the pixel""s signal output. For solid state detectors, the degrading effects due to lag generally increase with decreasing time between image acquisitions, and/or the number of consecutive images.
An individual pixel experiencing excessive lag may appear to have a significantly different signal than some or all of its neighboring pixels and exhibit an artifact. For example, pixels experiencing more lag than neighboring pixels may look brighter while pixels experiencing less lag than neighboring pixels may look darker, even though each of the pixels have received the same level of radiation. The artifacts may be any size or shape, for example, point artifacts, line segments, or rectangular regions. The artifacts may simply be bothersome to a clinician, or may potentially impact a patient diagnosis.
In the past, image lag has been characterized by the average lag of the detector. A detector with average lag falling within a predefined range was considered good, or acceptable for clinical use. A detector with average lag falling outside the predefined range was identified as not acceptable for clinical use and scrapped. Unfortunately, previous methods were unsuccessful at detecting and correcting individual pixels, lines, and other arbitrary artifacts due to lag uniformity problems.
Therefore, it is desirable to identify which pixels, because of excessive lag, present an unacceptable risk for exhibiting artifacts.
In accordance with at least one embodiment, a method is provided for identifying, in a digital x-ray detector, pixels that retain an amount of charge sufficient to cause an image artifact. In accordance with the method, a lag artifact threshold is obtained which identifies the amount of residual charge that will cause image artifacts. Optionally, the lag artifact threshold may be representative of a percent confidence that the pixel will not exhibit an image artifact. Alternatively, the lag artifact threshold may be obtained by using a perception value determined by perception studies utilized to identify image artifacts. The difference between the perception value and the percent confidence may also be utilized to obtain the lag artifact threshold.
The pixel lag experienced by a pixel in the detector is determined. Optionally, the pixel lag may represent the amount of residual charge held by a pixel. Alternatively, a median pixel lag may be computed based upon the amount of lag experienced by a group of pixels surrounding the pixel. In one instance, the group of pixels may define a region of pixels one square centimeter surrounding the pixel. It is possible that an excess pixel lag, representing the amount of lag that the pixel experiences which is greater than the median pixel lag, may be computed. Pixels retaining pixel lag in excess of the lag artifact threshold are then identified. Additionally, a noise value representative of the noise experienced by the detector may be identified. The noise value may be utilized, together with the radiation level, to which the detector was exposed, to calculate the excess pixel lag. The pixels experiencing lag in excess of the lag artifact threshold may be used to generate a lag pixel artifact map.
In accordance with an alternative embodiment, the digital x-ray detector may be exposed to a first radiation level. A first set of signals representing the first radiation level may be obtained. The detector may then be exposed to a second radiation level, and a second set of signals may be obtained. It is possible that the first radiation level is a high level, and the second radiation level is a low level. The lag artifact threshold may be based upon the first and second radiation levels. The amount of lag experienced by the pixel in excess of the average lag experienced by the surrounding pixels may be calculated using the two radiation levels.
In accordance with at least one embodiment, a method is provided for calculating a lag pixel artifact map for a digital x-ray detector. The detector is exposed to radiation. Then the detector is read consecutively to obtain at least first and second sets of pixel values. Optionally, the first and second sets of pixel values are acquired within a predetermined time of each other, such as 200 ms. A lag artifact threshold identifying the amount of residual charge held by pixels in the detector that may cause image artifacts is obtained. Alternatively, the lag artifact threshold may be representative of a percent confidence that the pixel will not exhibit an image artifact.
Excess pixel lag may also be calculated for a pixel in the detector. Optionally, a perception value determined by perception studies utilized to identify image artifacts may be used to calculate the excess pixel lag. The excess pixel lag may be calculated by utilizing a ratio of the retained charge on a current pixel and the average retained charged for a set of pixels surrounding the current pixel. Additionally, a noise value may be determined for the noise that is experienced by the detector. The noise value may be utilized, together with the radiation level, to which the detector was exposed, to calculate the excess pixel lag. The excess pixel lag is compared to the lag artifact threshold. If the excess pixel lag is greater than the lag artifact threshold, the pixel is added to a lag pixel artifact map.
By identifying which pixels in the digital x-ray detector may cause image artifacts due to excess lag, the image artifacts may be corrected. This is advantageous as the image artifacts may be distracting to the technician or radiologist who is viewing the x-ray image. Additionally, the presence of image artifacts may lead the radiologist to prescribe additional, unnecessary scans or tests to confirm that the image artifacts are not representative of patient anatomy. The additional scans add to the overall cost and are undesirable as the patient may be exposed to additional radiation.